CONTENTS ^aiWee A PERIODICAL OF PARTICLE PHYSICS FALL 1991 VOL. 21, NUMBER 3 FEATURES Editors RENE DONALDSON, BILL KIRK WHITE DWARFS Contributing Editor Going Gentle Into that Good Night MICHAEL RIORDAN A review of what we know and don't know about these planet-sized stars. Editorial Advisory Board Virginia Trimble JAMES BJORKEN, ROBERT N. CAHN, DAVID HITLIN, STEWART C. LOKEN, JOHN REES, RONALD RUTH 7 PARTICLE PHYSICISTS, TEACHERS MARVIN WEINSTEIN & SCIENCE EDUCATION REFORM Photographic Services Particlephysicists are teaming with teachers TOM NAKASHIMA to improve the quality of pre-college BETTE REED science teaching in the United States. Marjorie Bardeen Illustrations TERRY ANDERSON, KEVIN JOHNSTON 12 WORKSHOPS TARGET THE NEXT SYLVIA MACBRIDE, JIM WAHL LINEAR COLLIDER Distribution A review of recent linear collider conferences CRYSTAL TILGHMAN by two participants. David Burke and Ewan Paterson The Beam Line is published quarterly by the Stanford Linear Accelerator Center, P.O. Box 4349, Stanford, CA 94309. DEPARTMENTS Telephone: (415) 926-2585 BITNET: BEAMLINE@SLACVM FAX: (415) 926-4500 SLAC is operated by Stanford University under 17 DATES TO REMEMBER contract with the U.S. Department of Energy. The opinions of the authors do not necessarily 18 TOWARD THE NEXT LINEAR COLLIDER: reflect the policy of the Stanford Linear Accelerator Center. THE ACCELERATOR TEST FACILITY AT KEK Koji Takata FROM THE EDITORS' DESK Cover: A photograph ofa planetarynebula in Aquarius, 21 taken with the 200-in. telescope on Mt. Palomar. Planetary nebulae are the ejected outer layers of dying 22 LETTERS TO THE EDITORS stars of relatively low mass. The relic cores become the white dwarfs discussed in Virginia Trimble's article in this issue. Photo courtesy of California Institute of 23 CONTRIBUTORS Technology. »^4 WE HAVE MADE several attempts during the past two years to secure a spectrum of the companion of Sirius. .. .The great mass of the star, equal to of Sirius, and its low luminosity, one one- hundredth part of that of the Sun and one ten-thousandth part of that of Sirius, make the character of its spectrum a matter of exceptional interest.... The line spectrum of the companion is identical with that of Sirius in all respects so far as can be judged from a close comparison of the spectra.... Direct photographs taken by Dr. van Maanen with and without the use of a yellow color screen agree with the spectrographic results in indicating that the companion of Sirius has a color index not appreciably different from that of the principal star. valter Adams Alvan Clark "The Spectrum of the Companion of Sirius," Walter S. Adams, Publications of the Astronomical Society of the Pacific 27, 236-237 (1915) reproduced in A Source Book in Astronomy and Astrophysics, 1900-1975, Lang and Gingerich, Editors, Harvard, 1979. I Only a very few stars that begin (PV = nRT) thermal-gas pressure in (hence temperatures), and lumi- their lives with more than six to stars like the sun, but by electron nosities were nearly always strongly eight times the mass of our sun will degeneracy pressure in white dwarfs. correlated. The properties of Sirius end spectacularly. In other words, if B, reported in 1914 by Walter S. you allowed every star a vote , * then Adams at Mt. Wilson Observatory, white dwarfs would win, hands HISTORY defied the correlations. The star was down, but supernovae seem to have Friedrich Bessel (as in functions) blue (hot) and yet faint. A handful of much better public relations. advocated massive, dark companions additional white dwarfs joined in the Admittedly, they are easier to make as the cause of wiggles in the motion defiance over the next decades. sound exciting, being responsible for of Sirius and Procyon that he had Theoretical understanding began the production of heavy elements seen in data from a couple of decades with R.H. Fowler's development of like uranium and thorium (and the up to 1844. The companion of Sirius (non-relativistic) quantum statistics distribution of most of the rest), for advanced from dark to dim in 1862 in 1926 and was essentially com- accelerating cosmic rays, leaving through the intervention of Alvan plete when S. Chandrasekhar pre- pulsars behind, and (perhaps) trig- G. Clark, arguably the last optical sented the exact equation of state for gering new generations of star for- astronomer to make a fundamental relativistically degenerate electrons mation. Still, numbers should count discovery with a telescope he had and the properties of stars dominated for something, and white dwarfs have designed and built himself. * * Turn- by that degeneracy between 1930 the additional, parochial attraction ing a newly-ground 18.5-inch lens and 1935. His contemporaries were of showing us what our sun will toward the brightest star in the sky, slow to understand the correctness eventually become-unfortunately he saw separately the 0.01% of the and importance of what he had done, not in time to prevent the next round photons that come from the com- but I think this belongs more to the of presidential primaries, but only panion, 10 arc-sec away from the after five or six billion years. glare of Sirius A. Procyon B turned Briefly, a white dwarf contains up as a separate point of light in "Of course, if stars are allowed to vote, a 1894. then they should also pay taxes. One the mass of a star, crammed into dollar per star in our Milky Way galaxy volume the size of a planet, and has White dwarfs counted as a would just about take care of this year's therefore a density larger than that of problem, however, only after the on- budget deficit (which I decline abso- set of serious classification of stellar lutely to describe as "astronomical.") terrestrial substances by a factor of a " A point which may not be totally million. The force of gravity holding spectra and the recognition that irrelevant to recently revealed problems stars together is balanced by ordinary absorption line patterns, stellar colors in optical telescope design. 2 FALL 1991 realm of sociology than to history of and a good many others. Enough science. Incidentally, 20 years be- astronomers earn a living studying tween recognition of an astronomi- CVs to permit gatherings of a hundred cal problem and arrival at a solution or two every few years. that then holds up for much longer is Very occasionally, mass transfer a fairly typical time scale. or merging in white dwarf pairs may 0 gd0) trigger explosive burning of the car- t- bon and oxygen core material up to THINGS WE UNDERSTAND iron. The stars are blown to About one white dwarf per year smithereens called Type I superno- C) forms somewhere in our galaxy, a vae. (Massive stars make type II, com- Subrahmanhan Chandrasekhar number arrived at independently by moner but less bright.) counting the young (hot, relatively bright) ones, by counting prospective progenitors, and by counting the nebulae ejected in the intermediate The Great Scheme of Things "thin shell" phase (called planetary nebulae, and if you want to know AT LEAST BY THE STANDARDS OF ASTROPHYSICS, stellar structure and why, you must read a real astronomy evolution are in pretty good shape-meaning we can marshall a set of coupled, book). Ten percent or more of a non-linear differential equations and their solutions to support what is said in the next few paragraphs. complete stellar inventory consists Stars are, by definition, stable objects deriving their energy from nuclear of white dwarfs, just sitting there, reactions. A gas cloud of standard composition (about 3/4 hydrogen, 1/4 helium, radiating away the thermal (kinetic) and 2% everything else) and mass = 0.085-100 solar masses [one solar mass 33 energy of their carbon and oxygen (M0 ) = 2x10 g] will contract to form such a star. Smaller masses do not get hot nuclei from underneath very thin enough for any fusion reactions; bigger ones are unstable. All stars spend 90 skins of hydrogen and helium. percent or more of their lives fusing hydrogen to helium. The time to exhaust They 2 will continue this uneventful course hydrogen fuel is, very crudely, t = [10 yr/(M/M) ], that is, millions of years for big stars, billions for little ones, including the sun. until the universe recontracts, their Masses less than about 0.4 Me leave their helium cinders so degenerate that baryons decay, or they collapse to no further contraction, heating, or fusion can occur. The universe is not yet old black holes by barrier penetration. * enough for this to have happened to any isolated star. But stripping of outer layers Half or so of all stars have can occur among close stellar pairs, leaving low-mass helium cores in shorter companions close enough for gas to times. There exist low-mass white dwarf binaries where the stars are tearing each flow from one to the other. Material other apart viciously enough that the layers accessible to spectroscopy confirm the transferred to a white dwarf falls into mostly-helium composition predicted for these cores. Larger masses all heat a deep potential well, hence radiating to at least 108 K at their centers and so fuse helium to carbon and oxygen, in roughly equal amounts, first in their cores and then in thin copiously as it goes, and exploding shells. A combination of radiation pressure and pulsational instability in this "thin- from time to time, when it piles up shell" phase ejects much of the hydrogen-rich envelope. Thus initial masses up to to fusion temperatures and densities. 6 or 8 M0 are stripped down to carbon-oxygen cores of 0.4-1.4 M0 . These Collectively such systems are called degenerate C-O cores are much the commonest kind of stellar relic.